I am very very confused, help

How is it that I am always going to get my lead, always going to push that advantage heavily, and still 2/3 games are unwinnable? Sure I make a couple of mistakes a game, but a couple shouldn't autolose the game. Everyone first timing champs in ranked, they turbo int to ganks because they don't ward, I can't buy a sightstone early to ward people's lanes anymore, trackers is gone as well. I don't think I've had a bot lane win lane in like 100 games. There has to be something I can do better, but I've been playing at near the top of my game, and I don't believe in a system that forces 50% win rates, so can't possibly be that I won too many games and the system is punishing me. What can I be doing better?

Also how do you tell people who want to play sylas and kayle that they shouldn't without banning their champion?